The present invention relates to a process for synthesizing diamonds, and more particularly to an improvement in and/or relating to a system for heating raw materials in a reaction chamber for the synthesis of diamonds.
For synthesizing diamonds, there are two methods, namely the so-called direct method requiring only carbon as the raw material and the so-called indirect or catalytic method requiring as the raw materials carbon and a solvent-catalyst metal of cobalt, nickel or a like metal belonging to Group VIII of the periodic table or an alloy thereof. Both methods require substantially the same temperature condition of about 1,000.degree. C. but the former requires a very severe pressure condition in the order of 400 Kb whereas to the contrary, only a pressure of 45 to 55 Kb is required in the latter. Therefore, the present invention has particular, though not exclusive, reference to a system for heating raw materials in a reaction chamber for the synthesis of diamonds with use of the indirect method.
In order to heat the raw materials charged into a reaction chamber, there have been proposed three systems, i.e. the so-called direct system wherein electrical current is directly passed through the raw materials per se to heat the same by utilizing those inherent electrical resistances (U.S. Pat. No. 2,947,609 and British Pat. No. 1,049,182), the so-called indirect system wherein electrical current is passed through a heating element arranged around a reaction vessel defining the reaction chamber to heat the raw materials the use of heat radiated from the heating element (U.S. Pat. No. 3,082,477 and British Pat. No. 951,167) and the so-called semi-direct system wherein electrical current is partially passed directly through the raw materials per se to heat the same by utilizing those inherent electrical resistances and partly passed through a heating element arranged in a reaction vessel to define the reaction chamber and to heat also the raw materials with use of heat radiated from the heating element (U.S. Pat. No. 2,947,610 and British Pat. No. 830,210).
The direct heating system has the advantages that a heat generation coefficient or thermal efficiency to supplied electrical current is higher than that in the indirect heating system and an inner volume of the reaction chamber can be made relatively larger to increase the yield in each reaction, since there is no special heating element required. The direct heating system has the disadvantage that maintenance of a certain constant heating condition throughout the reaction chamber is quite difficult because it is difficult to provide a uniform flow of electrical current through the raw materials which makes the current density distribution in the reaction chamber uneven. Thus, the temperature of the central portion in the reaction chamber is apt to be higher than that of peripheral portions which causes a relatively large temperature gradient therein. Also, the electrical resistance in the reaction chamber varies as conversion of the raw material carbon substance to diamonds proceeds, to cause a local temperature variation.
The indirect heating system has an advantage in that, as a whole, the temperature in the reaction chamber can be kept at a constant level regardless of the reaction progress but has the disadvantage that the inner volume of the reaction chamber is smaller, because the heating element must be arranged around the reaction vessel and upper and lower ends of the reaction vessel should be closed by a disk plate of electrical and thermal insulation material.
The semi-direct heating system is a combination of the direct system and the indirect system but has the disadvantage that the life of each punch for generating a high pressure in the reaction chamber becomes shorter since the heat generated in the reaction chamber is apt to be transmitted directly to each end portion of the oppositely arranged punches so as to generate the required high pressure in the reaction chamber.
Therefore, an object of the present invention is to provide a new indirect system for heating a reaction chamber, which obviates and overcomes the disadvantage encountered in the conventional indirect heating system as referred to without spoiling any advantage to be enjoyed in the conventional indirect heating system.
According to one of the aspects, the present invention is to provide a process for synthesizing diamonds by utilizing the new indirect heating system.
According to the invention, the process is attained by steps of charging a raw material containing at least material carbon into a reaction chamber defined by a thin tubular heating element and a pair of disk plates of a thermal and electrical insulation material and arranged in the heating element, generating required high pressure in the reaction chamber and applying electrical current to the heating element to indirectly heat the raw material in the reaction chamber.
According to another aspect, the present invention is to provide the new indirect heating system per se for carrying out the process.
According to the present invention, the system comprises a hollow cylindrical reaction vessel made of a refractory material, a thin tubular electrically heating element is fitted in the reaction vessel so as to circumferencially define the reaction chamber therein, a pair of disk plates each made of a thermal and electrical insulation material having a diameter substantially the same with the inner diameter of the heating element so as to be fitted therein to close the open end thereof and longitudinally define the reaction chamber, said reaction chamber being filled with a raw material which contains at least material carbon, a pair of electrically conductive disks, each electrically contacting the edge of said heating element, a pair of electrically conductive rings each arranged to contact with each of said electrically conductive disks to supply electrical current to said heating element, and a pair of heat insulation disks each fittedly arranged in each of said electrically conductive rings to prevent heat generated in the reaction chamber from leaking.
The heating element may be of a metal having a relatively high melting point, for instance 1,200.degree. C. or more, such as iron, cobalt, nickel, molybdenum, tantalum or the like, or of a non-metal material such as graphite or the like. The disk plate is prepared by a material having a good electrical insulation property at a high temperature, such as magnesia, alumina, silica or a like oxide, or boron nitride or a like nitride.
According to one of embodiments of the invention, the space of reaction chamber may be partitioned with one or more disk plates made of the same material as the heat and electrical insulation disk plate disposed at both ends of the heating element. The disc plates divide the reaction chamber into two or more compartments which substantially completely inhibit the resistance heating of the raw material charged in each of said compartments.
When a diamond synthesis is carried out, in usual cases, the reaction chamber is previously filled with the raw materials in one of the following manners.
(a) a cylindrical block of a solvent-catalyst metal is arranged in the central portion of the reaction chamber and then two cylindrical blocks of graphite are charged in the reaction chamber to fulfill the a space therein to hold the metal block in a tight fashion.
(b) contrary to (a), a cylindrical graphite block is arranged in the central portion of the reaction chamber and then two cylindrical blocks of a solvent-catalyst metal are charged in the reaction chamber to fulfil a space therein to hold the graphite block in a tight fashion,
(c) a graphite disk block is arranged at each end of the reaction chamber and the remaining space is filled with a mixture of graphite powder and solvent-catalyst metal granules,
(d) a hollow cylindrical graphite is fitted in the reaction chamber and then a rod of a solvent-catalyst metal is fitted into the graphite cylinder,
(e) graphite disk plates and solvent-catalyst metal disk plates are alternatively arranged to fulfil the reaction chamber (this is preferable, in view of the fact that the raw materials charged to the reactor can be made larger to attain a higher yield and the contacting area between the graphite and solvent-catalyst metal can be made larger) and the like.
In the manner of (e), further, diamond seed crystals may initially be arranged between each of said graphite solvent-catalyst metal disk plates, in order to obtain better quality diamond crystals. In this case, please note that no desired effect can be attained unless each of the diamond seed crystals has a diameter larger than 10.mu., since smaller diamonds will disappear during a mutual diffusion between the material carbon such as graphite and the solvent-catalyst metal when the reaction chamber is heated to a temperature causing the conversion reaction of graphite to diamonds. This phenomenon of seed diamond disappearance or fusion can be prevented by arranging a separation disk plate between each graphite disk plate or diamond seed crystals and solvent-metal disk plate, said separation disk plate being of a thin metal plate having no affinity to carbon, such as copper, silver, gold or an alloy thereof. The separating plate serves to prevent an initial mutual diffusion between the graphite and solvent-catalyst metal to permit the effective use of diamond seed crystals having a diameter smaller than 10.mu..